Recently, the developers have nerfed the in game textures of objects in the game to make the game easier to run and it is highly suspected that it is because the game is starting to struggle.
Lindsay talks about the engine somewhat in her Q&A with James Turner, however, It's important to keep in mind the background with how the game was created when it was "Project Olympus" and when there's mention of "what they could have done differently" and "adding flexibility" there was only so much they could do because of the limitations of the engine.
(source: https://www.youtube.com/watch?v=SKzjiiox0Fk)
Edit: Another great video in reference to "Project Olympus"
https://www.youtube.com/watch?v=epxTbdLxc7o
